1. Field of the Invention
The present invention relates generally to arrangements for minimizing the propagation of vibrational energy and resonant mode behavior and, in particular, to a loudspeaker enclosure having a constrained layer damping system for minimizing the propagation of vibrations and controlling the resonant modes of the enclosure.
2. Description of the Relevant Art
A loudspeaker operates by converting electrical energy into vibrational (sound) energy using one or more transducers. Transducers transmit vibrational energy through their frame, which is usually made of metal or plastic, into the walls of the loudspeaker cabinet. This energy will propagate freely throughout the cabinet, exciting panel resonances (i.e., the energy is amplified at specific frequencies) and then reradiate into the air. This re-radiation of energy is undesirable because it can be perceived as distortion and coloration of the primary signal in the frequency range of 200 Hz to 1 kHz. In addition, these re-radiation points or resonant modes act as undesired phantom sound sources, compromising the sound field imaging capabilities of the loudspeaker.
Several approaches have been taken by manufacturers to address this problem, including: (1) decoupling the transducer from the cabinet front baffle by using a "soft" mounting system; (2) adding internal bracing and otherwise increasing wall rigidity to increase the frequency of panel resonant modes; (3) adding extensional damping materials and compounds to the interior surfaces of the cabinet walls to damp vibrational energy; and (4) casting the front baffle from concrete or similar energy absorbing material.
Decoupling the transducer from the cabinet is an undesirable approach because it prevents the transducer from utilizing the overall mass of the loudspeaker cabinet to minimize unwanted motion of the transducer frame. If the transducer is decoupled from the cabinet, there will be relative movement between the transducer frame and the cabinet, resulting in a loss in perceived fidelity. This loss of perceived fidelity is particularly noticeable in the low level detail of the reproduced sound.
The addition of internal bracing and stiffening of the cabinet walls is a desirable solution because it pushes the panel resonant modes to higher frequencies where they cause less audible damage. Unfortunately, this method alone is not enough--the resonant modes still exist at higher frequencies.
The addition of extensional damping materials or compounds to the interior walls of the cabinet also improves performance of the speaker, provided that the extensional damping material applied actually works. In most cases, the extensional damping material used is not effective, except for damping small amounts of high frequency vibration. The thickness and composition of the extensional damping material used is critical in determining its effectiveness. It is also critical that at least 50% of the surface area of the interior walls be covered for the extensional damping material to be effective.
Casting a front baffle of the loudspeaker from an acoustically "dead" material, such as concrete, offers an effective solution, but not without its own set of problems. Such an approach must deal with the complication of how to attach such a heavy and massive baffle to the rest of the loudspeaker cabinet without compromising the mechanical integrity of the overall structure. Moreover, the heavy baffle usually requires attachment using gaskets and screws in order to keep the enclosure airtight and, thus, cannot maintain the mechanical rigidity of the baffle. In any case, this approach still does not address the damping of the rest of the loudspeaker cabinet walls.